The present invention relates to electrical measurements and, in particular, to probe cards.
In testing small electrical devices (e.g., integrated circuits and circuit boards), it is common to use probe cards that provide an interface between test instruments and the device under test (DUT). The probe card provides the connections between probe needles and the test instruments/cables. In turn, the probe needles make electrical connections with the DUT during testing. By substituting probe cards with differing probe layouts, it is possible to change where on a DUT tests are made. A common configuration for probe cards is have the probe needles arranged radially about the DUT with connection pathways radiating outward from the probe needles. The opposite ends of the connection pathways then provide connection points for the test instruments/cables. Typically, the connection pathways are either integral with or mounted on a supporting structure along with the probe needles (e.g., a printed circuit board).
Besides the touch down locations of the needles, it is often important to consider the nature of the required connections to the DUT. For example, in the case of very low current DC measurements, minimizing extraneous leakage current is often important, while in AC measurements, losses and reflections due to impedance mismatches are often important.
These considerations often result in having to change probe cards depending on the type of measurement to be made, even if the probe needles themselves are in the correct location, the connection pathways between the instrument connections and the probe needles may not be suitable for the measurement.
Referring to FIG. 14, a probe card 1 suitable for low current testing connects the source measure unit 2 (SMU 2) to the probe needle 3 and the SMU 4 to the probe needle 5. A SMU can source a voltage/current and measure a current/voltage, respectively. The probe needles 3, 5 provide test connections to the DUT. As is typical in low current DC measurements, guard conductors 6, 7 containing a “guard” voltage are provided next to the signal conductors 8, 9, respectively. The guard voltage is typically supplied by the respective SMU 2, 4. This voltage is a buffered version of the signal on the respective signal conductor. Because the guard voltage and the signal voltage are equal, the signal conductors 8, 9 do not “see” any potential difference to cause leakage from the signal conductors 8, 9. It is said that this “guards” the signal conductors from leakage. Therefore, the guard conductors 6, 7 are often simply called guards. They are often coaxial with the signal conductors, but other configurations are also used, for example, such as being a planar conductor near to the signal conductor while having a width substantially greater than that of the signal conductor or other strip line configurations suitable for printed circuit board implementation. The ground connection 10 between the SMUs 2, 4 is shown off the probe card 1, but may also be on the probe card 1.
Referring to FIG. 15, the probe card 1 can be used to connect AC test instrumentation 11, 12 to the DUT. However, this configuration will often not be satisfactory for AC measurements as the guard conductors 6, 7 are “floating” at the AC voltage. From a transmission line perspective, the guard conductors 6, 7 are transmission line stubs as they near the DUT. The return path for the AC signals is through the ground connection 10, not through the ends of the guard conductors 6, 7. The configuration is unlikely to exhibit the measurement systems' characteristic impedance, instead causing losses and reflections of the AC signal. If the AC signal is such that this transmission line effect is significant (e.g., radio frequencies), then a different probe card configuration will be needed for AC measurements.